This invention relates to actuator systems having dual drive path ball screws and more particularly to a secondary drive path ball screw having a rod end swivel.
There are many components of an aircraft that are positioned by operation of a ball screw actuator. In such an actuator, there is relative rotation between a ball screw and a ball screw nut to achieve linear translation of the ball screw nut. One of the actuator elements is fixed and the other actuator element is connected to a flight control surface. Because of use of a dual drive ball screw actuator in controlling critical surfaces for aircraft flight, it is common to construct the ball screw of inner and outer tubular members which are interconnected at their ends. The inner tubular member provides a primary drive path for the ball screw and the outer tubular member provides a secondary drive path to be used in the event of a failure in the primary drive path. This structure is known as a dual drive actuator.
In the primary drive path, the inner tubular member is rotated. The inner tubular member has a helical ball groove which is associated with the ball screw nut by means of balls positioned in a portion of the helical groove so that rotation of the inner tubular member will cause the linear translation of the ball screw nut. The ball screw nut, in turn, is connected to an output tube which is connected to a rod end by a rod end swivel. The rod end is connected to the flight control surface to be actuated. So as ball screw translates linearly so does the output tube, rod end swivel and the rod end which then moves the flight control surface.
In the secondary drive path, the ball screw nut is rotated while locking the primary screw, which in turn rotates and linearly translates the output tube. However, rod end cannot be rotating. Therefore a rod end swivel is used to remove the rotation so that the rod end only moves linearly. FIG. 1, shows a typical prior art rod end swivel assembly 1. The assembly 1 includes a housing 2 mounted about the output tube 3. A thrust shaft 4 is mounted for rotation with the output tube 3 by a series of needle bearings 5 and thrust bearings 6. Mounted to the thrust shaft is the rod end shaft 7. These bearings remove the rotation so that the rod end only movers linearly.
Because dual load path ball screws are often used in flight control applications, it is imperative that their weight and size be kept as small as possible. A disadvantage to the rod end swivel shown in FIG. 1 is that to handle the thrust load that can be generated in certain aerospace applications, the rod end swivel becomes too large and too heavy.
Accordingly, a need exists for a rod end swivel that can handle the larger thrust loads encountered in some aerospace applications without becoming too large and/or too heavy.
An object of this invention is to provide a dual drive ball screw having a rod end swivel that for a given thrust load is smaller and lighter that prior art dual drive ball screws.
The present invention achieves this object providing a drive path ball screw having a ball screw shaft coupled at one end to a gear train and coupled at the other end to a ball screw nut. The ball screw shaft and ball screw nut are coupled through complementary helical grooves filled with ball bearing balls. The outer surface of the ball screw nut is splined and can be driven by a second gear train. An output tube is coupled at one end to the ball screw nut and at its opposite end has a connector portion with a groove on its inner surface. A rod end has a coupling member for coupling to a surface to be actuated and a shaft portion with a groove on its outer surface. This shaft portion is received with in the connector portion and the corresponding grooves are coupled through the use of a plurality of ball bearings. This arrangement for coupling the rod end to the connector portion, referred to as a rod end swivel, improves the performance of the ball screw over the prior art ball screws that use thrust and needle bearings.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.